1. Field of the Invention
The present invention relates generally to quadrature voltage controlled oscillators (VCOs), and more specifically to quadrature VCOs covering a wide frequency range and compensating for far-end phase errors.
2. Prior Art
Over the last few years, there has been significant expansion in the need for wireless local area networks (WLAN). Traditionally, network connectivity was provided through wires running through the walls, floors or ceilings of the area requiring network connectivity. Such connectivity provides certain advantages, but also has certain limitations, such as the need to place physical cables and the need to replace cables as modifications in the networks are required, just to name a few. Therefore WLANs (wireless local area networks) are becoming increasingly attractive for use, as radio frequencies (RF) have the ability, although limited at times, to overcome physical obstacles and allow for a significantly more flexible design of local networks.
While WLAN has its very well recognized advantages, it also suffers from problems common to high frequency wireless technologies. Physical obstacles can reduce the quality of service (QoS) to below desired levels, due to obstruction, reflection, noise, and other dynamic characteristics of the environment in which the WLAN system operates. In many cases, the environment in which the WLAN operates is constantly changing, and requires that the WLAN adapts to these changes and continues to provide a desired level of QoS.
Due to the limitations of the environment in which WLAN operates, it is necessary that the system functions in a manner that provides the best performance possible, i.e., provides the ability to transfer the maximum data possible, or in other words, provide the highest possible bandwidth on both the receiving and transmitting ends.
The data is superimposed on an RF signal, using for example orthogonal frequency division multiplexing (OFDM) as the modulation technique. The RF modulation involves modulation of both the amplitude and the phase angle. Phase modulation and demodulation is achieved by, for example, the use of a pair of mixers that utilize a quadrature local oscillator (LO). The quadrature oscillator provides an in-phase signal ‘I’ and a quadrature phase ‘Q’, both being in the same frequency. However, the ‘Q’ signal is delayed by 90 degrees with respect of the ‘I’ signal. It is well-known in the art that the more accurate the 90 degree shift between the ‘I’ and ‘Q’ signals, the greater the amount of data that can be sent on the carrier signal.
It is further noteworthy that the phase error may occur at a distance from the generation of the ‘I’ and ‘Q’ signals. This happens due to the high frequencies used in WLAN, over 1 GHz, and therefore, while the ‘I’ and ‘Q’ signal may be sufficiently accurate at the generation point, they may still be insufficiently accurate at the point-of-use. If it is desired to achieve a high data rate, it is essential that the phase error not exceed a few degrees of error from the desired 90 degree level.
It is therefore important to design the circuits in a way that will reduce the level of error, and preferably be self-correcting. It would be further desirable that the system be able to generate the ‘I’ and ‘Q’ signals such that at the point-of-use, the accuracy will be the highest. It would be further preferable if the solution provided be insensitive to manufacturing process variations and tolerances of integrated components so as to ensure the operation of the quadrature VCO within the requirements for high performance.